Manage your subscription

Dark matter no-show puts WIMPs in a bind

By Lisa Grossman

Working well… if only the WIMPs would show up

(Image: LUX Dark Matter)

Despite tantalising early hints of a sighting, the most sensitive search yet for dark matter has come up empty. First results from the Large Underground Xenon (LUX) detector in South Dakota, announced today, failed to confirm previous potential sightings reported by other detectors. That may spell trouble for elegant recent theories of a shadow universe where myriad particles interact via their own dark forces.

Dark matter is the invisible stuff thought to make up about 80 per cent of the universe’s matter, and that gives away its presence only by exerting a gravitational tug on ordinary matter. The most popular dark matter candidates are weakly interacting massive particles (WIMPs). So far unseen, these would also interact with normal matter via the weak force, so should smack into it every so often in a way that can be detected.

WIMP detectors are usually placed underground to shield them from background particles. Earlier this year, two such instruments in a Minnesota mine reported hints of WIMPs with a mass of between 1 and 10 gigaelectronvolts. The results back up intriguing models which say dark matter could be akin to normal matter in that it could be made of many kinds of particles, including low-mass WIMPs.

Advertisement

Dead end

Hopes were high that the LUX detector would confirm the hits. Buried almost 1500 metres deep in a former South Dakota gold mine, LUX scans for signs of dark matter interacting with a third of a tonne of liquid xenon. If the previous strikes were genuine, this highly sensitive detector should have seen one every 80 minutes, on average.

But the first results of three months of hunting, announced at the Sanford Lab in Lead, South Dakota, find no firm evidence of dark matter, although they confirm that the detector is as sensitive as planned. “We’ve seen nothing better than anyone else,” quips team member Dan McKinsey of Yale University. That’s bad news for the apparent whiffs of low-mass WIMPs in Minnesota.

“If LUX performs as well as we hope it will and they don’t see anything, I think this window will be killed,” Dan Hooper at Fermilab in Batavia, Illinois, told New Scientist before the announcement.

Wimpier WIMPs

“I’m afraid that these light WIMP models for us seem to be dead,” says LUX team member Richard Gaitskell of Brown University in Rhode Island.

There could yet be light at the end of the mineshaft. LUX is designed to search across a range of masses and may find signs of heavier dark matter. It could also flesh out recent models that suggest WIMPs are even wimpier than assumed, in that they are less able to interact with normal matter.

“LUX is a machine that can look over a very broad set of dark matter candidates, and that’s what we want,” says McKinsey. “It’s a fishing expedition, but we have a big net.”